The present invention generally relates to the field of voltage regulators, and in particular to voltage regulator systems utilized to control the charging of a battery, such as the battery in a vehicle.
In prior voltage regulator systems for vehicles it is known to provide a voltage regulator which senses battery voltage and provides a pulse width modulated output signal that varies in duty cycle in accordance with the difference between sensed battery voltage and a reference signal. This output signal is used to control a power switching device connected in series with a field coil across the battery voltage potential. The field coil controls excitation of stator windings of a voltage generating system which, after rectification of the output of the stator windings, provides a charging signal for the battery. Such voltage regulator systems, as described above, are conventional and well understood.
In some of the above-noted voltage regulator systems, it is necessary to provide a relatively large voltage, larger than battery voltage, at a control terminal of the power switching device so as to insure that maximum field current is provided when the switching device is on. In some cases an FET (field effect transistor) is utilized as the power switching device, with the drain electrode connected to battery voltage and the source terminal is connected through the field coil to ground potential. In such a configuration, to provide a gate voltage in excess of battery voltage when it is desired to have the FET on, a prior voltage regulator system has utilized a voltage doubler circuit. In the prior voltage doubler circuit, a voltage of approximately twice battery voltage is selectively provided at the gate terminal to insure that approximately the entire battery voltage potential is applied across the field coil when the FET is on.
The voltage doubler circuit of the prior voltage regulator relies on utilizing the pulse width modulated output signal of the voltage regulator to selectively couple and decouple a large magnitude capacitor across the battery voltage potential via a switch device. The end result is that essentially a voltage doubler is provided by the capacitor and the selective switching of the capacitor across the battery voltage potential. However, in such a configuration, a very large capacitance for the capacitor is utilized to insure that at high duty cycle percentages of the voltage regulator output signal, the voltage across the capacitor does not substantially decrease during the long on-duty cycle and thereby decrease the magnitude of the voltage at the gate electrode of the FET. When the prior voltage regulator produced an output signal which essentially resulted in the FET being on all of the time since field coil current was required all of the time, additional complex logic circuitry was required to provide some alternate coupling and decoupling of the capacitor across the battery voltage, and this increased the circuit cost. Without this additional logic circuitry, the twice battery voltage to be maintained at one terminal of the large capacitance capacitor would decrease due to leakage effects and the loading of the turned-on FET device. This is undesirable as it would result in reducing field coil current when the voltage regulator has indicated that maximum field coil current should be provided. Also, the required large magnitude capacitor is expensive, and it cannot be implemented as part of an integrated circuit so as to reduce circuit cost.